Optical waveguide modulators used in high-speed optical communications, such as those based on a Mach-Zehnder (MZ) interferometer (MZI), may be implemented in a photonic chip in the form of a photonic integrated circuit (PIC). The photonic chip may be based on a semiconductor material such as silicon (Si), indium phosphate (InP), or the like, which enables to utilize well-developed semiconductor manufacturing technologies and approaches to transmission and control of optical signals and high-speed electrical signals. An MZI-based waveguide modulator may require active control of their bias setting, which determines at which point of a transmission transfer characteristic the MZI operates during modulation. The bias setting of an MZI may be controlled by varying a refractive index of a waveguide arm of the MZI to control a relative phase of interfering light waves in the MZI in the absence of modulation signals.
Very high speed optical systems may benefit from using advanced quadrature modulation (QM) formats such quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM). Optical QM signals may be obtained using an optical IQ modulator, which may be implemented using nested MZI structures. Such structures typically require controlling several bias voltages. For example, a QPSK optical signal may be generated by combining two BPSK (binary phase shift keying) optical signals in quadrature, i.e. with a phase shift therebetween ϕIQ equal to 900, or π/2 radians (rad). The optical signals being combined, conventionally referred to as the in-phase (I) and quadrature-phase (Q) signals, may be generated by splitting light from a suitable light source between two MZ modulators (MZM) driven by two NRZ electrical data signals, conventionally referred to as the “I” and “Q” electrical data signals, and then combining their outputs in quadrature. An optical BPSK signal may be generated by applying a binary AC electrical modulation signal to an ideal MZM that is biased at its minimum transmission point. When optical outputs of two such MZMs are coherently added together in quadrature, a QPSK optical signal results.
In semiconductor-based optical modulators, the biasing is typically done using resistive heaters disposed to locally heat a portion of a waveguide arm of a respective inner MZM or outer MZI, and is controlled by superimposing a small AC dither signal over a DC bias voltage and sensing a dither signature in an optical output. The bias settings of the two inner MZMs may be controlled by monitoring respective dither signals at the output of the inner MZMs or at the output of the IQ modulator, with the latter typically being a preferred choice as it allows simplifying the control circuitry. The drift of a bias voltage VIQ that controls the IQ phase shift ϕIQ in the outer MZI away from its optimal setting may also be monitored based on a feedback from the modulator's output. However, the output optical signal may be distorted when conventional dither-based techniques of bias monitoring are applied for non-ideal semiconductor-based MZMs characterized by a finite extinction ratio (ER).